1. Field of the Invention
The present invention relates to radio communication systems, and more particularly to a radio communication system which employs the code division multiple access (CDMA). More specifically, the present invention is concerned with a spread spectrum communication receiver which receives spread spectrum signals transmitted by a transmitter in which data to be transmitted is code-division multiplexed using an orthogonal code and is spectrum-spread using the sequence of a PN code (spreading code).
2. Description of the Related Art
Generally, data to be transmitted over a plurality of channels is divided and multiplexed before transmission. Examples of the above procedure are the FDM (Frequency Division Multiplex) system, the TDM (Time Division Multiplex) system, and the CDM (Code Division Multiplex) system.
In the CDM system, channels are separately defined by performing an orthogonal conversion using respective orthogonal codes which are spread in the same time-frequency space. It is easily possible to determine and modify, for each channel, the data rate and weighting. For this reason, the CDM system is suitable for hierarchical transmission.
In the field of broadcasting, there has been considerable activity in the development of a digital video signal transmission system in which the channels defined by the CDM system are assigned different weighting values on the transmitter side and a graceful degradation process is carried out on the receiver side, In the graceful degradation process, the channels to be combined are selected according to qualities of the received signals.
In the field of mobile communications, the IS-95 system which is standardized as a CDMA cellular telephone system utilizing direct sequence (DC) spread spectrum is known. In the IS-95 system, a control channel and communication channels are defined by the CDM system. On the transmitter side, control information and speech information are assigned to channels defined by using the orthogonal codes and are then transmitted, On the receiver side, a RAKE process is carried out in which one channel is subjected to a demodulation process using a plurality of fingers (also referred to as branches). The RAKE process is directed to improving the quality of received signals.
A further description will be given of the rake receive process. The RAKE process is inherent in the spread spectrum communication system and enables path diversity reception.
In digital communications such as the spread spectrum communication system, a transmission wave arrives at the receiver, as a direct wave and reflected waves. That is, the receiver directly receives the transmission wave, and receives waves reflected by buildings or the like. Generally, there are many routes (multipath) in which the transmission wave is reflected and then arrives at the receiver. Hence, the receiver receives many signals which propagate along the different radio paths. Such a group of received signals propagated through different (isolated) paths can be called a received multipath signal. The received signals that arrive at the receiver via the different paths have different propagation delay times. Hence, the quality of received signals may decrease due to the interference caused by multipath.
The PN code allocated for the spectrum spreading process at the transmitter is also allocated in order to perform a spectrum despreading process at the receiver. If the PN code allocated at the receiver has a time offset to the received signals propagated along the paths, the cross-correlation will be destroyed. With the above in mind, the following process is employed at the receiver in order to avoid a decrease in the quality of the received signal due to the interference caused by multipath. A despreading part of the receiver gives the PN code a phase offset dependent on the propagation delay time. The despreading part despreads the received multipath signal and produces a despread signal of one path of the received multipath signal which has the propagation delay time corresponding to the given phase offset. That is, the received signals of the other paths of the received multipath signal do not be despread because of less correlation. In this manner, the PN codes are given the phase offsets for the respective received signals of the paths, which can be despread by the respective PN codes given the respective phase offsets. Hence, the received multipath signal can be despread for each isolated path.
Thus, the receiver is equipped with despreading parts provided in parallel. The despreading parts give the PN code the phase offsets corresponding to the propagation delay times of the respective paths. The PN codes thus obtained are allocated in order to despread the received multipath signal, so that the despread symbols can be separately obtained.
The received symbols thus obtained are respectively weighted and are combined at a combining part of the receiver, so that combined demodulated symbols having good quality can be obtained.
As described above, the RAKE process selectively demodulates the received multipath signal having the signals propagating along a plurality of paths, and then combines demodulated symbols. Thus, the RAKE process realizes the path diversity reception.
Generally, a fixed number of channels is allocated to each user in the above-mentioned digital video signal transmission system and the CDMA cellular telephone system. Thus, the demodulator of the receiver is designed to perform the demodulating operation on the given number of channels.
Recently, it has been required that the mobile communications be used to provide not only services directed to transferring speech and low-data-rate data but also advanced services directed to transferring data at a high data rate. Since the conventional system has the fixed number of channels for each user, it is necessary to increase the data rate per channel in order to meet the above requirement. However, in practice, an increase in the data rate is limited to a certain level, and thus the conventional spread spectrum communication system cannot sufficiently realize the high-data-rate data transfer.
Further, a particular attention should be drawn, in the mobile communications, to fading which affects the received multipath signal every moment and degrades the quality of received signal. Data may be lost by fading.